The present invention concerns the fabrication of integrated circuits and pertains particularly to a method for preventing micromasking in shallow trench isolation process etching.
In fabricating integrated circuits, various processes are used to form field oxide. For example, a local oxidation of silicon (LOCOS) process is often used to form field oxide regions. In a LOCOS process, a layer of pad oxide is formed. On top of the pad oxide, a layer of nitride is formed. The nitride is patterned and etched. Field oxide is grown on the substrate at places where the nitride has been etched to expose the substrate. The nitride and pad oxide are then removed.
Shallow trench isolation (STI) is gradually replacing conventional LOCOS process for the formation of an isolation structure as technology is evolving to submicron geometry. STI has various advantages over the conventional LOCOS process. For example, STI allows for the planarization of the isolation structure. This results in better control of critical dimension (CD) when defining a gate stack of a transistor. Better control of CD when defining the gate stack results in better control of CD in further processing steps which occur after the gate stack is defined.
During conventional STI process, a buffer oxide/nitride mask is patterned onto the silicon wafer to define the trench area. Before a trench etch is performed, a cleaning is performed with wet chemicals such as sulfuric acid and hydrogen peroxide, typically ending with a spin-rinse-dry (SRD). The trench area is etched and filled by chemical vapor deposition (CVD) with a fill oxide so that, for an average trench width, local planarization between the trench and the nitride mask is achieved. The fill oxide is then polished back by chemical mechanical polishing (CMP) or etched back by dry plasma etch until it is flush with the nitride layer. The nitride mask is then removed, leaving the fill oxide extending above the trench. The fill oxide is then further removed and a planar surface is provided. This further removal of the fill oxide is done by dry or wet plasma etching of the fill oxide.
Unfortunately, SRD tools are prone to leaving water spots which can induce micromasking. The micromasking can result in a leakage site resulting in defective parts.